Wafer inspection refers to inspecting a semiconductor wafer for abnormalities or defects present on the surface of the wafer. These defects could affect the functionality of integrated circuits (ICs) fabricated on the wafer, leading to decreased production yield of ICs. Detecting defects, identifying their root cause, and eliminating them is of foremost importance in semiconductor fabrication.
The sizes of individual components inside ICs have been decreasing with every new generation of semiconductor technology in order to improve performance while reducing cost, a trend widely known as Moore's law. As components in ICs become smaller, tiny defects that were previously overlooked as being too small to affect IC performance begin to manifest themselves as killer defects that could bring down production yield. Consequently, every next generation technology node comes with the challenge of detecting continually shrinking defect sizes.
Simultaneously, the diameter of wafers used by semiconductor fabs has been increasing in order to accommodate an increasing number of ICs on a single wafer for saving cost. When combined with the decreasing nature of defect sizes, the above mentioned increasing wafer diameters presents next generation semiconductor wafer inspection tools the daunting challenge of detecting continually decreasing defects sizes on a continually increasing surface area.
Traditional dark-field wafer inspection tools illuminate a laser spot on the surface of a wafer and use collection optics with a high numerical aperture to detect scattered radiation. While the width of the laser spot is typically in the order of micrometers, the diameter of the wafer can be as large as 450 mm. In order to cover the entire surface of the wafer, the spot is sequentially scanned to illuminate different regions of the wafer until the entire water is covered. Traditional dark-field wafer inspection tools employ a finite number (typically, less than 5) of photodetectors to detect scattered light.
In traditional dark-field wafer inspection tools, it is difficult to inspect a large area of a wafer at a given time. This is because of two reasons: 1) spot size of laser beam is small, 2) the field of view of collection optics is small. While (1) may be addressed by expanding beam size, addressing (2) is challenging because of the need to have a large numerical aperture to capture light scattered at a wide range of angles. Designing collection optics having a large field of view and a large numerical aperture is a formidable task. Collection optics with large numerical aperture also imposes the constraint of reduced working distance between wafer and collection optics, leading to tight optomechanical tolerances.
A trade-off between inspection throughput (measured in wafers per hour) and defect sensitivity exists in traditional wafer inspection tools. The reason for this trade-off is because defect sensitivity is related to the total energy scattered by a defect. Total scattered defect energy can be modeled by multiplying scattered optical power from defect with the amount of time the spot spends on the defect. Scattered power from defect is proportional to the intensity of illumination on defect. Any attempt to increase defect sensitivity by decreasing spot size (so as to increase illumination intensity) or increasing the amount of time the spot spends on defect directly affects throughput. Reducing spot size increases the number of points the spot needs to traverse on the wafer, thereby increasing overall scan time per wafer. Increasing the amount of time a spot spends on a defect by reducing scanning speed also increases the overall scan time for the wafer. Therefore, in traditional wafer inspection tools, increased defect sensitivity comes at the price of decreased inspection throughput.
Traditional dark-field wafer inspection tools suffer from a number of disadvantages, including: a) low throughput due to two-dimensional scanning; b) complex collection optics due to large numerical aperture; c) reduced defect identification capabilities due to limited number of photodetectors; d) trade-off between numerical aperture and working distance; e) limited field of view; f) trade-off between throughput and defect sensitivity; and g) complex scanning mechanism due to two-dimensional scanning requirement.
Accordingly, there is a need for an improved wafer inspection system that improves wafer throughput; simplifies collection optics; increases defect identification capabilities; decouples trade-off between numerical aperture and working distance; improves field of view; relaxes trade-off between throughput and defect sensitivity; and simplifies scanning mechanism for covering entire wafer surface.